Method for controlling the set time of cement

ABSTRACT

An oil and gas well cementing composition and method are shown utilizing hydraulic cement, water a cement hydration retarder and an additive which is effective over a selected time period to degrade the retarder. The additive is a controlled release oxidizer which releases nascent oxygen over time to gradually oxidize the cement hydration retarder, thereby providing a single cement slurry design for cementing pipes for oil and gas wells where wide temperature differentials exist between the two ends of the pipe.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to cementing compositions and methods forcementing oil and gas wells and, more specifically, to method andcomposition for cementing pipes for oil and gas wells where the pipeshave opposing ends and the temperature differential between the two endsof the pipe within the well borehole is as much as 75° F., or more.

2. Description of the Prior Art:

Hydraulic cements are cements that can set under water. This settingproperty serves a variety of purposes. Hydraulic cements are often usedin placement of pipes or casings within a well bore of a subterraneanformation for the construction of oil and gas wells. In the oil and gasindustry, successful cementing of well pipe and casing during oil andgas well completion requires cement slurries having several importantproperties. The slurry must have a pumpable viscosity, fluid losscontrol, minimized settling of particles and the ability to set within apractical time. Polymers, such as hydroxyethyl cellulose,carboxymethylhydroxyethyl cellulose, polyvinyl alcohol andpolysulfonated polymers are commonly used to provide these importantproperties.

In a typical completion operation, the cement slurry is pumped down theinside of the pipe or casing and back up the outside of the pipe orcasing through the annular space. This seals the subterranean zones inthe formation and supports the casing. The amount of water used informing the cement slurry depends upon the type of hydraulic cementselected and the job conditions at hand. The amount of water used canvary over a wide range, depending upon such factors as the requiredconsistency of the slurry and upon the strength requirement for aparticular job.

These completion procedures often place the hydraulic cement within ornext to a porous medium, for example, earthern strata in the well bore.When this happens, water tends to filter out of the slurry and into thestrata during placement and setting of the cement. Many difficultiesrelate to an uncontrolled fluid loss of this type, such as anuncontrolled setting rate, premature gelation of the slurry, bridging ofthe annular space between the formation and the casing, improperplacement of the slurry, impaired strength properties and acontamination of the surrounding strata. These conditions are allundesirable in oil and gas well cementing operations. Special additiveshave consequently been designed to control fluid loss during wellcementing operations.

To achieve a satisfactory primary cementing job, it is also important toachieve a tight bond between the pipe or casing and the cement sheath toprevent vertical communication of fluids or gas along or within thecement column. In order to achieve a tight bond, it is important toreduce the cement matrix permeability and retain water during theinitial set, effectively blocking the porous cement structure.

In cementing certain long strings of pipe or casing, the temperaturedifferential between the two ends of the pipe might be 75° F., or more.In such cases, a slurry design must be adequate to cement a pipe in agas or oil well where the two ends of the pipe are, for example, at 310°F. and 235° F., respectively. One problem presented by such a widetemperature differential is that the slurry is required to remain in apumpable state, for example, for two hours (job time plus safety time)at the lower, relatively high temperature zone (310° F. bottom holecirculating temperature, BHCT, in this example), and yet developsufficient compressive strength at the upper, relatively low temperaturezone (235° F. in this example) to minimize the rig time. For purposes ofthis example, the requirements are pumping time of about 2 hours 30minutes±15 minutes at 310° F. BHCT and 16100 psi pressure at a 16150foot depth, while developing a compressive strength of at least 200 psiin twelve hours. The slurry design must also be adequate to cement twosuch well zones at very wide temperature differentials through the useof a single slurry, rather than through the use of staged cementslurries.

Traditional API Class "H" cement systems would generally not be able toachieve these desired results for several reasons. Typical long stringsof pipe of the type under consideration use slurries whose strengthdevelopment can be somewhat slow, or even non-existent, when retardedfor bottom hole conditions and cured at lower temperatures.Additionally, given the long exposure to annular conditions experiencedby the lead portion of the cement column being pumped, fluid losscontrol is a necessity. Since many of the fluid loss additives tend toretard cement setting times and strength development (especially whenloaded for bottom hole circulating temperature and then cured undersurface conditions), the addition of fluid loss additives only serves toexaggerate the nature of the problem.

Thus, a need exists for an improved well cementing composition whichprovides a slurry having a pumpable viscosity, adequate fluid losscontrol, minimized settling of particles and the ability to set within apractical time and develop sufficient compressive strength, even wherewell zones at wide temperature differentials are being cemented.

A need also exits for such an improved cement composition which allowsthe transition from a hydrostatic pressure transmitting liquid to a setcement, with very little, if any, time spent in the plastic,self-supporting/gas-migrating stage.

SUMMARY OF THE INVENTION

The cementing method and compositions of the invention are used tocement well zones at a wide temperature differential, e.g. 75° F. ormore, with a single slurry. The cementing compositions of the inventionare comprised of hydraulic cement, water present in an amount sufficientto produce a pumpable slurry, a cement hydration retarder and aneffective amount of an additive which serves to gradually oxidize thecement hydration retarder over a selected period of time, wherebydestruction of the retarder serves to accelerate the setting process ofthe cement.

The cement hydration retarder can be any of a number of traditionalretarders such as those selected from the group consisting oflignosulfonates, organic carboxylates and mixtures thereof.

The additive can be any controlled release oxidizer which does nototherwise detract from the desirable properties of the slurry whichreleases nascent oxygen over a selected period of time, the release ofnascent oxygen serving to gradually oxidize the cement hydrationretarder over the selected period of time. The controlled releaseoxidizer can be selected from the group consisting of alkaline earth andzinc salts of peroxide, perphosphate, perborate and percarbonate andmixtures thereof and can include, for example, calcium peroxide, calciumperphosphate, calcium perborate, magnesium peroxide, magnesiumperphosphate and zinc perphosphate. The controlled release oxidizer canalso include such oxidants as calcium hypochlorite, magnesiumhypochlorite, chloramine T, trichloroisocyanuric acid,trichloromelamine, dichloroisocynaurate dihydrate, anhydrousdichloroisocynaurate and mixtures thereof. The controlled releaseoxidizer can also be an encapsulated oxidizer such as an encapsulatedpersulfate.

The cementing compositions of the invention can be used to cement a pipewithin a well bore which has a top, relatively low temperature zone anda bottom, relatively high temperature zone and wherein the period oftime over which the oxidizer releases nascent oxygen is selected tomaintain the cementing slurry in a pumpable state for at least two hoursat the relatively high temperature zone. The compositions of theinvention are designed to develop a compressive strength of at least 200psi in twelve hours at the top, relatively low temperature zone of thewell bore. The compositions are pumped to the desired location in thewell bore and allowed to harden to a solid mass.

Additional objects, features and advantages will be apparent in thewritten description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of compressive strength versus time which compares aprior art cement slurry with a slurry of the invention; and

FIG. 2 is a graph of compressive strength versus time, similar to FIG.1, which compares a different prior art slurry with a slurry of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an improved cementing composition and themethod of using the cementing composition. The cementing composition iscomposed of hydraulic cement, a cement hydration retarder, an additivecomprising a controlled release oxidizer and water.

In this disclosure, the term "hydraulic cement" refers to any inorganiccement that hardens or sets with water. The term "hydraulic cement" alsoincludes cements having minor amounts of extenders such as bentonite,gilsonite, and cements used either without any appreciable sand oraggregate material or such cements admixed with a granular fillingmaterial such as sand, ground limestone, fly ash, and the like.Hydraulic cements, for instance, include Portland cement, aluminouscements, pozzolan cements, and the like. Thus, for example, any of theclass "A-H" and "J" cements as listed in the API Spec 10 First Edition,January 1982 are suitable for the invention. Strength enhancers such assilica powder can be employed as well.

Mixing water with the dry hydraulic cement compositions produces aslurry. A sufficient amount of water should be added to the hydrauliccement to form a pumpable slurry of suitable consistency. The water canbe any type of water commonly used in the cement industry, such as freshwater, brines, sea water, and the like. The water can have additionaladditives present, for instance, potassium chloride, dispersants,accelerators, viscosifiers, retarders and accelerators. API Spec 10First Edition, January 1982, which is known in the cement industry,describes an approved apparatus and method for measuring the consistencyof cement slurries in terms of Bearden units of consistency (Bc). Apumpable slurry should measure in the range from about 2 to 20 Bc andpreferably be in the range of about 5 to 11 Bc. Slurries thinner than 5Bc tend to have greater particle settling and free water generation.Slurries thicker than about 11 Bc become increasingly difficult to pump.

Depending upon the particular slurry and intended conditions of use, theamount of mixing water in the slurry of the present invention rangesfrom about 30 to 150 weight percent based upon the dry weight of cementand preferably is in the range of about 35 to 90 weight percent.

The cementing compositions of the invention also preferably include anorganic cement hydration retarder. Such retarders will be familiar tothose skilled in the oil and gas well cementing industries. The mostcommon retarders used are the lignosulfonates such as calcium and sodiumlignosulfonate. Other retarders include lignin-liquid, carboxymethylhydroxyethyl cellulose (CMHEC) and borax. The lignosulfonates aregenerally effective up to circulating temperatures of about 200° F. Forhigher temperatures, special retarder blends containing CMHEC, such as"DIACEL LWL," are effective for bottom hole circulating temperatures of350° F., or higher.

Additionally, many traditional fluid loss additives, such as thosecontaining CMHEC also act to retard the setting time of cement. Forpurposes of the present invention, the term "retarder" will beunderstood to encompass both traditional "retarders", such as thelignosulfonates, and traditional "fluid loss" additives which themselvesretard, such as those containing CMHEC, hydroxyethyl cellulose (HEC) andAMPS copolymers (see, e.g., U.S. Pat. No. 4,632,186, issued Dec. 30,1986, "Well Cementing Method Using An AM/AMPS Fluid Loss AdditiveBlend." The preferred retarders of the invention are the lignosulfonatesand the retarders containing organic carboxylate functionalities, suchas those containing CMHEC. The lignosulfonate and organic carboxylateretarders are generally present in the cementing compositions of theinvention in the range from about 0.01 to 2.00% by weight, based on theweight of dry cement, most preferably in the range from about 0.10 to1.50% by weight.

The cementing compositions of the invention also have incorporatedtherein an effective amount of an additive which serves to graduallyoxidize the cement hydration retarder over a predetermined period oftime, whereby destruction of the retarder serves to accelerate thesetting process of the cement.

Preferably, the additive is a controlled release oxidizer which releasesnascent oxygen over a selected period of time, the release of nascentoxygen serving to gradually oxidize the cement hydration retarder overthe selected period of time. The controlled release effect can beachieved in different ways, including the use of a substantiallyinsoluble oxidant instead of a coated soluble oxidant, e.g., apersulfate. Thus, the controlled release can be effected by selecting asuitable sparingly soluble or substantially insoluble material as theoxidizer, the solubility rate of the material serving to determine therate at which more nascent oxygen enters the cement slurry to therebydegrade the retarder and/or fluid loss additive. Such controlled releaseoxidizers can be selected from the group consisting of alkaline earthand zinc salts of peroxide, perphosphate, perborate and percarbonate andmixtures thereof, and can include, for example, calcium peroxide,calcium perphosphate, calcium perborate, magnesium peroxide, magnesiumperphosphate and zinc perphosphate.

The substantially insoluble oxidant is preferably selected from thegroup consisting of alkaline earth and zinc salts of peroxide,perphosphate, perborate and percarbonate. The rate of solubility isdependant upon several parameters such as the solubility productconstant, the temperature of the surrounding formation, pH of the fluidand the surface area of the granules. Generally, suitable oxidants forpurposes of the present invention will have a solubility of less than 1%by weight, such as 1 gram material/100 grams water.

Representative examples of the substantially insoluble oxidants of theinvention include:

    ______________________________________                                                   Solubility in grams per 100 cc                                                Cold Water                                                                            Hot Water Other Solvents                                   ______________________________________                                        calcium peroxide                                                                           sl s      --        s a                                          CaO.sub.2                                                                     calcium perphosphate                                                                       i         i         i a                                          Ca.sub.2 P.sub.2 O.sub.8                                                      calcium perborate                                                                          sl s      --        s a, NH.sub.4 salts,                         Ca(BO.sub.3).sub.2               sls ac a                                     magnesium peroxide                                                                         i         i         s a                                          MgO.sub.2                                                                     magnesium    i         i         s, NH.sub.4 salts,                           perphosphate                     i liq NH.sub.3                               Mg.sub.2 P.sub.2 O.sub.8                                                      zinc perphosphate                                                                          i         i         s a, NH.sub.4 OH;                            Zn.sub.2 P.sub.2 O.sub.8         i al                                         ______________________________________                                         where                                                                         i = insoluble                                                                 sl s = slightly soluble                                                       s a = soluble in acid                                                         i a = insoluble in acid                                                       sl s ac a = slightly soluble in acids and alcohol                        

In terms of the above table, "substantially insoluble" will be taken tomean a compound of the above type having a solubility in cold and hotwater of "i" or "sl s" as listed in the Handbook of Chemistry andPhysics, 72nd Edition, CRC Press.

One particularly preferred control release oxidant is a calcium peroxideavailable commercially from FMC Corporation as "PERMEOX."

Although the above listed sparingly soluble or substantially insolubleoxidizers are adequate for practicing the present invention, otheroxidants have been successfully used, as well, including such oxidantsas calcium hypochlorite, magnesium hypochlorite, chloramine T,trichloroisocyanuric acid, trichloromelamine, dichloroisocynauratedihydrate, anhydrous dichloroisocynaurate and mixtures thereof.

The controlled release oxidizer can also be an encapsulated oxidizersuch as a coated persulfate of the type familiar to those skilled in theart. One such encapsulated oxidizing accelerator is described in issuedU.S. Pat. No. 5,164,099, issued Nov. 17, 1992 to Cooney et al., nowassigned to the assignee of the present invention, the disclosure ofwhich is incorporated herein by reference. The Cooney patent describesan encapsulated "breaker" for fracturing fluids which can also be usedfor purposes of the present invention as a controlled release oxidizerfor a cement slurry. A percarbonate, perchlorate or persulfate, such ascalcium persulfate, is encapsulated with a polyamide membrane which ispermeable to a fluid, in this case the water in the cement slurry, suchthat the persulfate diffuses through the membrane to act upon the cementhydration retarder to degrade the retarder with time. The encapsulatedbreaker is commercially available as the "ULTRA PERM CRB " breaker fromBJ Services Company of Houston, Tex. The encapsulated oxidizer ispreferably present in the range from about 0.01 to 5.0%, most preferablyabout 0.10 to 0.50% by weight, based upon the weight of dry cement.

The following non-limiting examples are illustrative of the invention.

In Table I which follows, slurries used in the evaluation of thecontrolled release oxidizer of the invention were mixed in a "WARING"blender, following Section 5 of API Spec 10. Procedures used to measurefluid parameters such as fluid loss (API FL), thickening time (TT) andrheology were obtained from API Spec 10 also.

The base slurry evaluated was a Joppa Class "H" oil field cement havingblended therein 35% by weight of dry cement of "SF-3" (silica flour),35% by weight of cement hematite (weighing material), 37.2% by weight ofwater (BWOW) of salt, 0.5% by weight of cement "CF-22" fluid lossadditive (grafted AMPS-tannin copolymer), 0.10% by weight of cement"AF(S)" tributylphosphate defoamer, 0.10% by weight of cement "ASA 301"viscosifier and free water reducer. The ingredients were mixed with44.2% by weight water and conditioned as described in API Spec 10. Theslurry had a density of 18.8 pounds per gallon (ppg). In Table I whichfollows, thickening time was measured at 310° F., fluid loss at 180° F.and compressive strength was measured with "ULTRASONIC CEMENT ANALYZER"("UCA") compressive strength (CS) at 235° F. "WR-15" is a medium to hightemperature retarder containing approximately 70% silica flour, 15%sodium gluconate and 15% of a lignosulfonate organic acid blendcommercially available from BJ Services Company, "WZ 499792" is thecalcium peroxide controlled release oxidizer.

                                      TABLE I                                     __________________________________________________________________________    "WR-15"/PEROXIDE SYSTEM                                                       Base Slurry: Joppa H + 35% "SF-3" + 35% HEMATITE + 37.2% Salt (BWOW) +        0.5% "CF-22" + 0.10% "AF(S)" + 0.10% "ASA 301" + 44.2% Water                  D = 18.8 ppg; TT at 310° F.; FL loss at 180° F.; 2 at           235° F.                                                                                       API FL                                                 %          %           Loss                    UCA CS at 235° F.       "WR-15"    "WZ 499792"                                                                          TT   (ml/ Rheology (rpm) RT/180° F.                                                                 50 psi                                                                             500                                                                                Final PSI            Run                                                                              Date                                                                             (BWOC)                                                                             (BWOC) (Hr:Min)                                                                           30 min)                                                                            600 300                                                                              200                                                                              100                                                                              6  3  (Hr:Min)                                                                           (Hr:Min)                                                                           (Hr:Min)             __________________________________________________________________________    1  2/25                                                                             1.25 0.10   2:12 30   860/                                                                              492/                                                                             353/                                                                             207/                                                                             31/                                                                              23/                                                                              11:11                                                                              11:41                                                                              6378                                             434 236                                                                              164                                                                              89 10 8            (24:36)              2  2/26                                                                             1.40 0.15   2:30                         10:52                                                                              13:00                                                                              2978                                                                          (approx.                                                                      250 psi @                                                                     12:00)               3  2/24                                                                             1.25 --     2:13                          21:00+                        4  2/26                                                                             1.40 0.15   2:19 34   968/                                                                              538/                                                                             380/                                                                             215/                                                                             27/                                                                              19/                                                                              11:21                                                                              11:56                                                                              3013 (13:23)                                     509 271                                                                              180                                                                              97 10 6            Free                                                                          water -                                                                       trace                5  2/28                                                                             1.40 0.15   2:21 32   1037/                                                                             622/                                                                             457/                                                                             265/                                                                             46/                                                                              35/                                                                              10:04                                                                              11:10                                                                              8798 (32:54)                                     343 188                                                                              129                                                                              70 8  6            Free                                                                          water -                                                                       1.5 mL               6  3/1                                                                              1.30 0.10   2:37 34   1037/                                                                             572/                                                                             392/                                                                             219/                                                                             27/                                                                              15/                                                                               8:15     Free                                             438 223                                                                              150                                                                              77 8  4            water -                                                                       5                    __________________________________________________________________________                                                             mLs              

As shown in Table I, the addition of the calcium peroxide controlledrelease oxidizer slowly releases nascent oxygen which oxidizes thelignosulfonate and organic carboxylates present in the WR-15 retarderover time to enhance the development of compressive strength, especiallyat the higher, relatively low temperature end of the pipe beingcemented. The controlled release of the oxidizer also maintains theslurry's pumpable state for at least the desired two hours pumping time.This allows the almost complete destruction of the retarder within 3-4hours, thereby accelerating the setting process and eliminating problemswith "over-retarding" on jobs involving long strings of pipe with widewell bore temperature differentials, as was sometimes the case in thepast. Note particularly Runs 1 and 3. The development of 50 psicompressive strength in Run 3, without the controlled release oxidizer,took twenty-one plus hours at 235° F. To develop the same 50 psicompressive strength took only eleven hours, eleven minutes at 235° F.with the oxidizer additive of the invention included in the same baseslurry.

FIG. 1 is a graph of compressive strength versus time which wasdeveloped using an ultrasonic cement analyzer. Curve 3B represents atraditional slurry made with Holnam Class "H" cement, 35% S-8 (silicaflour), 0.4% R-8 retarder (lignosulfonate and organic acid blend) andcontaining 50% water. Curve 2B is the same slurry but with the additionof 0.1% by weight of cement of calcium peroxide oxidizer. Thetemperature differential was from 270° F. BHCT/320 BHST to 250° F.,where BHCT is bottom hole circulating temperature and BHST is bottomhole static temperature. The thickening time was 5 hours 58 minutes forRun 3B and 4 hours 55 minutes for Run 2B.

FIG. 2 is similar to FIG. 1 the slurries having the same ingredients butwith a temperature differential of 270° F. BHCT/320 BHST to 230° F.Slurry 7B has added thereto 0.1% by weight of cement of calciumperoxide.

As can be seen from FIGS. 1 and 2, the addition of a controlled releaseoxidizer results in a markedly improved rate of strength development,once development has started.

In a method of using the cementing composition, a pumpable slurry ismade by adding together hydraulic cement, water, a cement hydrationretarder and the additive described above to formulate the cementingcomposition of the invention. The pumpable slurry is then pumped to adesired location within a well bore, where it is allowed to harden.

An invention has been provided with several advantages. The cementingcompositions of the invention can be used to cement a pipe within a wellbore which has a top, relatively low temperature zone and a bottom,relatively high temperature zone and wherein the period of time overwhich the oxidizer releases nascent oxygen is selected to maintain thecementing slurry in a pumpable state for at least two hours at therelatively high temperature zone. The compositions of the invention aredesigned to develop a compressive strength of at least 200 psi in twelvehours at the top, relatively low temperature zone of the well bore.

While the invention has been shown in only one of its forms, it is notthus limited but is susceptible to various changes and modificationswithout departing from the spirit thereof.

What is claimed is:
 1. A method of cementing an oil and gas well, themethod comprising the steps of:forming a cementing slurry by combiningdry hydraulic cement, water in a sufficient amount to produce a pumpableslurry, a cement hydration retarder and an additive comprising acontrolled release oxidizer which is effective to slowly release nascentoxygen over a selected period of time; pumping the cementing slurry to adesired location within the well bore; and allowing the cementing slurryto harden to a solid mass, the controlled release oxidizer serving togradually oxidize the cement hydration retarder over the selected periodof time.
 2. The method of claim 1, wherein the cement hydration retarderis selected from the group consisting of lignosulfonates, organiccarboxylates and mixtures thereof.
 3. The method of claim 1, wherein thecontrolled release oxidizer is selected from the group consisting ofalkaline earth and zinc salts of peroxide, perphosphate, perborate,percarbonate and mixtures thereof.
 4. The method of claim 1, wherein thecontrolled release oxidizer is an encapsulated oxidizer.
 5. The methodof claim 1, wherein the controlled release oxidizer is selected from thegroup consisting of:calcium peroxide, calcium perphosphate, calciumperborate, magnesium peroxide, magnesium perphosphate, zinc perphosphateand mixtures thereof.
 6. The method of claim 1, wherein the controlledrelease oxidizer is selected from the group consisting of calciumhypochlorite, magnesium hypochlorite, chloramine T, trichloroisocyanuricacid, trichloromelamine, dichloroisocynaurate dihydrate, anhydrousdichloroisocynaurate and mixtures thereof.
 7. The method of claim 1,wherein the cement hydration retarder is a traditional fluid lossadditive having a hydroxyl or carboxylate functionality.
 8. The methodof claim 7, wherein the fluid loss additive is selected from the groupconsisting of carboxymethylhydroxyethyl cellulose, hydroxyethylcellulose, AMPS copolymers, and mixtures thereof.
 9. The method of claim1, wherein the cementing slurry is used to cement a pipe within a wellbore which has a top, relatively low temperature zone and a bottom,relatively high temperature zone and wherein the period of time overwhich the oxidizer releases nascent oxygen is selected to maintain thecementing slurry in a pumpable state for at least two hours at therelatively high temperature zone.
 10. The method of claim 9, wherein thecementing slurry is designed to develop a compressive strength of atleast 200 psi in twelve hours at the top, relatively low temperaturezone of the well bore.
 11. A method of cementing a pipe having opposingpipe ends within a well bore of an oil and gas well where a largetemperature differential exists in the well bore adjacent the opposingpipe ends, the method comprising the steps of:forming a cementing slurryby combining dry hydraulic cement, water in a sufficient amount toproduce a pumpable slurry, a cement hydration retarder and an additivecomprising a controlled release oxidizer which is effective to slowlyrelease nascent oxygen over a selected period of time; pumping thecementing slurry within an annular area created between the pipe and thesurrounding well bore; and allowing the cementing slurry to harden to asolid mass, the controlled release oxidizer serving to gradually oxidizethe cement hydration retarder over the selected period of time, wherebydestruction of the retarder serves to accelerate the setting process ofthe cement.
 12. The method of claim 11, wherein the temperaturedifferential between the two ends of the pipe within the well boreholeis at least 75° F.
 13. The method of claim 11, wherein the cementhydration retarder is selected from the group consisting oflignosulfonates, organic carboxylates and mixtures thereof.
 14. Themethod of claim 11, wherein the controlled release oxidizer is selectedfrom the group consisting of alkaline earth and zinc salts of peroxide,perphosphate, perborate and percarbonate.
 15. The method of claim 11,wherein the controlled release oxidizer is an encapsulated oxidizer. 16.The method of claim 11, wherein the controlled release oxidizer isselected from the group consisting of:calcium peroxide, calciumperphosphate, calcium perborate, magnesium peroxide, magnesiumperphosphate, zinc perphosphate and mixtures thereof.
 17. The method ofclaim 11, wherein the controlled release oxidizer is selected from thegroup consisting of calcium hypochlorite, magnesium hypochlorite,chloramine T, trichloroisocyanuric acid, trichloromelamine,dichloroisocynaurate dihydrate, anhydrous dichloroisocynaurate andmixtures thereof.